Fuel cell system and method of controlling same

ABSTRACT

A fuel cell system includes a fuel cell configured to generate electric power with a fuel gas and an oxygen gas fed to the fuel cell and to discharge exhaust gas including CO 2  as a result of generating the electric power; a CO extraction part configured to reduce the CO 2  in the exhaust gas fed to the CO extraction part to CO, the CO extraction part including a processing container fed with the exhaust gas and a CO 2  adsorbing member provided in the processing container and formed of an oxide having an oxygen deficiency; and a CO recycling part configured to feed the extracted CO to the fuel cell as part of the fuel gas.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based upon and claims the benefit of priorityof the prior Japanese Patent Application No. 2009-063182, filed on Mar.16, 2009, and the Japanese Patent Application No. 2009-277796, filed onDec. 7, 2009, the entire contents of which are incorporated herein byreference.

FIELD

A certain aspect of the embodiments discussed herein is related to afuel cell.

BACKGROUND

In these years, fuel cells have drawn attention as a technology for theprevention of global warming. Unlike heat engines, fuel cells arebelieved to be able to convert chemical energy directly into electricalenergy, achieve high electric power generation efficiency, and reduceenergy consumption.

Solid oxide fuel cells (SOFCs) and molten carbonate fuel cells (MCFCs)are known as fuel cells capable of generating electrical power on alarge scale. These fuel cells have high reaction temperatures, but mayuse natural gas, coal gas, and CO (carbon monoxide) gas in addition tohydrogen gas as fuel.

Therefore, in SOFCs, attempts have been made to employ hydrocarbon gasand CO gas generated from coal and hydrocarbons such as methane (CH₄)gas generated by fermenting organic waste as source gases.

On the other hand, such SOFCs or MCFCs have CO₂ (carbon dioxide)contained in their exhaust gas. Accordingly, various proposals have beenmade to improve energy efficiency and reduce CO₂ emission in order toprevent global warming due to CO₂ emission.

For example, Japanese Laid-open Patent Publication No. 04-065066proposes reusing CO₂ included in exhaust gas by reducing CO₂ to ahydrocarbon or alcohol by electrolysis or photoelectrolysis. JapaneseLaid-open Patent Publication No. 2004-192824 proposes the technique ofreusing CO₂ by reducing CO₂ to a hydrocarbon such as methane usingbacteria such as methanogen.

SUMMARY

According to an aspect of the invention, a fuel cell system includes afuel cell configured to generate electric power with a fuel gas and anoxygen gas fed thereto and to discharge an exhaust gas including CO₂ asa result of generating the electric power; a CO extraction partconfigured to reduce the CO₂ in the exhaust gas fed thereto to CO, theCO extraction part including a processing container fed with the exhaustgas and a CO₂ adsorbing member provided in the processing container andformed of an oxide having an oxygen deficiency; and a CO recycling partconfigured to feed the extracted CO to the fuel cell as a part of thefuel gas.

The object and advantages of the embodiments will be realized andattained by means of the elements and combinations particularly pointedout in the claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and notrestrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a schematic diagram illustrating a fuel cell system accordingto a first embodiment;

FIG. 2 is a flowchart illustrating a CO₂ adsorption and decompositioncycle in the fuel cell system of FIG. 1 according to the firstembodiment;

FIG. 3 is a diagram illustrating a CO₂ adsorption and decomposition tankin the fuel cell system of FIG. 1 according to the first embodiment;

FIG. 4 is an enlarged view of a CO₂ adsorbing member used in the CO₂adsorption and decomposition tank of FIG. 3 according to the firstembodiment;

FIG. 5 is a timing chart of operations of a first exhaust gas processingsystem and a second exhaust gas processing system in the fuel cellsystem of FIG. 1 according to the first embodiment;

FIG. 6 is a flowchart illustrating an operation of the fuel cell systemof FIG. 1 according to the first embodiment;

FIG. 7 is a diagram illustrating a CO₂ adsorbing member according to asecond embodiment;

FIG. 8 is a diagram illustrating a CO₂ adsorbing member according to avariation of the second embodiment;

FIG. 9 is a diagram illustrating an operating state of the fuel cellsystem according to a third embodiment;

FIG. 10 is a diagram illustrating an operating state of the fuel cellsystem according to the third embodiment;

FIG. 11 is a diagram illustrating an operating state of the fuel cellsystem according to the third embodiment;

FIG. 12 is a diagram illustrating an operating state of the fuel cellsystem according to the third embodiment;

FIG. 13 is a diagram illustrating a fuel cell system according to afourth embodiment; and

FIG. 14 is a diagram illustrating an algae culturing tank used in thefuel cell system of FIG. 13 according to the fourth embodiment.

DESCRIPTION OF EMBODIMENTS

According to the proposal by Japanese Laid-open Patent Publication No.04-065066, however, CO₂ is reduced to the extent of a hydrocarbon byelectrolysis so that a large amount of energy is necessary to reduceCO₂. Therefore, this is not necessarily effective in reducing the amountof energy consumption in light of energy balance.

Further, the proposal by Japanese Laid-open Patent Publication No.2004-192824 requires a repository for garbage, a fermentation tank,and/or a gas purifier to reduce CO₂. This results in a complicatedsystem configuration and also causes a problem in that special attentionis necessary for maintenance of the fermentation tank or the like.

Preferred embodiments of the present invention will be explained withreference to accompanying drawings.

[a] First Embodiment

FIG. 1 is a schematic diagram illustrating a fuel cell system 100according to a first embodiment.

Referring to FIG. 1, the fuel cell system 100 includes a fuel cell 30.The fuel cell 30 is a solid oxide fuel cell (SOFC) or a molten carbonatefuel cell (MCFC).

The fuel cell 30 includes an air electrode (anode) 30 ₁, a fuelelectrode (cathode) 30 ₂, and an electrolyte 30 ₃ held (sandwiched)between the air electrode 30 ₁ and the fuel electrode 30 ₂. If the fuelcell 30 is an SOFC, an oxide such as stabilized zirconia is used as theelectrolyte 30 ₃. If the fuel cell 30 is an MCFC, a fused salt of alithium carbonate or sodium carbonate is used as the electrolyte 30 ₃.In practice, it is often the case that multiple cells each having theelectrolyte 30 ₃ held between the air electrode 30 ₁ and the fuelelectrode 30 ₂ are stacked through separators (not graphicallyillustrated) and used as a fuel cell.

The fuel cell 30 generates electric power by having the air electrode 30₁ fed with an oxidation gas such as air through a filter 35 and an airline 35 a and having the fuel electrode 30 ₂ fed with fuel such asnatural gas or petroleum gas through a filter 34 and a fuel line 34 a.

In the fuel cell 30, in the case of feeding methane CH₄ as a fuel gasand feeding air as an oxidation gas, the following reaction with oxygenin the air occurs so that electric power is generated:

CH₄+2O₂→CO₂+2H₂O.  (1)

In the case of using an SOFC as the fuel cell 30, generally, thisreaction is caused to occur at a high temperature of 800° C. to 1000° C.Further, in the case of using an MCFC as the fuel cell 30, this reactionis also caused to occur at a high temperature of 600° C. to 700° C.

On the other hand, in the SOFC or MCFC, carbon monoxide (CO) may also beused as fuel. In this case, desired power generation is performed by thefollowing reaction with oxygen in the air:

CO+1/2O₂→CO₂.  (2)

In this case also, the power generation reaction is caused to occur atas high temperatures as described above. The above-described reactions(equations) (1) and (2) may be caused to occur simultaneously.

As is seen from the above-described equations, in the SOFC or MCFC, CO₂is generated as a reaction product with use of natural gas, petroleumgas, or a gas containing carbon, such as carbon monoxide. The generatedCO₂ is discharged from the fuel cell 30 in the form of high-temperatureexhaust gas.

The generated high-temperature exhaust gas has energy. Therefore, thefuel cell system 100 of FIG. 1 has a heat exchanger 31 ₁ and a heatexchanger 31 ₂ provided in the air line 35 a and the fuel line 34 a,respectively, and feeding the high-temperature exhaust gas to the heatexchanger 31 ₁ and the heat exchanger 31 ₂, thereby heating the air fedthrough the air line 35 a and the fuel gas fed through the fuel line 34a before the air and the fuel gas are injected into the fuel cell 30.

The exhaust gas cooled through the heat exchangers 31 ₁ and 31 ₂ is ledto an H₂O separator 32 such as a cold trap, where water is separated.The H₂O separator 32 is provided with a valve V_(H), which isperiodically opened to discharge the separated water outside the fuelcell system 100.

Further, the fuel cell system 100 of FIG. 1 includes a CO extractionpart that extracts CO gas from the exhaust gas composed principally ofhigh-temperature CO₂ discharged from the fuel cell 30. The CO extractionpart reuses the CO gas thus extracted as fuel. As a result, the fuelcell system 100 of FIG. 1 achieves high energy efficiency.

A description is given below of the CO extraction part of the fuel cellsystem 100.

Referring to FIG. 1, the CO extraction part includes CO₂ adsorption anddecomposition tanks 40 ₁ and 40 ₂ fed with the exhaust gas of the fuelcell 30. The exhaust gas from which H₂O has been separated in the H₂Oseparator 32 is fed to the CO₂ adsorption and decomposition tanks 40 ₁and 40 ₂ through a valve Vf₁ and a valve Vf₂, respectively.

The CO₂ adsorption and decomposition tanks 40 ₁ and 40 ₂ extract carbonmonoxide (CO) from CO₂ in the exhaust gas. The CO gas thus extracted iscollected and stored in a CO storage tank 33 through a pump 36 ₁ and avalve Ve₁ and through a pump 36 ₂ and a valve Ve₂. The CO gas collectedinto the CO storage tank 33 is injected as additional fuel into the fuelgas fed to the fuel cell 30 through the filter 34 at an injection part33A provided in the fuel line 34 a, and is thus recycled.

FIG. 2 is a flowchart illustrating an overview of a CO₂ adsorption anddecomposition cycle in the CO₂ adsorption and decomposition tanks 40 ₁and 40 ₂. FIG. 3 is a diagram illustrating a configuration of the CO₂adsorption and decomposition tank 40 ₁. FIG. 4 is an enlarged view of aCO₂ adsorbing member 20 ₁ of the CO₂ adsorption and decomposition tank40 ₁. The CO₂ adsorption and decomposition tank 40 ₂ has the sameconfiguration as the CO₂ adsorption and decomposition tank 40 ₂.Accordingly, a description of the CO₂ adsorption and decomposition tank40 ₂ is omitted.

First, referring to FIG. 3, the CO₂ adsorption and decomposition tank 40₁ includes a container 10 ₁ of, for example, stainless steel, copper, ora nickel alloy and has the CO₂ adsorbing member 20 ₁ contained in theprocessing space of the container 10 ₁. Further, the CO₂ adsorption anddecomposition tank 40 ₁ has an exhaust gas feed part 11 ₁ formed in thecontainer 10 ₁. The exhaust gas feed part 11 ₁ is formed of a passagefor feeding the exhaust gas from the fuel cell 30 to the CO₂ adsorbingmember 20 ₁. Further, the CO₂ adsorption and decomposition tank 40 ₁includes a heat exchanger 12 ₁. The heat exchanger 12 ₁ heats the CO₂adsorbing member 20 ₁ to a predetermined temperature with the exhaustgas from the fuel cell 30.

The CO₂ adsorbing member 20 ₁ adsorbs CO₂ molecules in the fed exhaustgas, and decomposes them into carbon monoxide molecules and oxygenatoms. That is, in the CO₂ adsorbing member 20 ₁, the decomposition andreduction reaction expressed by the following equation occurs:

CO₂→CO+1/2O₂.  (3)

Likewise, the CO₂ adsorption and decomposition tank 40 ₂ includes acontainer 10 ₂ of, for example, stainless steel, copper, or a nickelalloy and has a CO₂ adsorbing member 20 ₂ contained in the processingspace of the container 10 ₂. Further, the CO₂ adsorption anddecomposition tank 40 ₂ has an exhaust gas feed part 11 ₂ formed in thecontainer 10 ₂. The exhaust gas feed part 11 ₂ is formed of a passagefor feeding the exhaust gas from the fuel cell 30 to the CO₂ adsorbingmember 20 ₂. Further, the CO₂ adsorption and decomposition tank 40 ₂includes a heat exchanger 12 ₂. The heat exchanger 12 ₂ heats the CO₂adsorbing member 20 ₂ to a predetermined temperature with the exhaustgas from the fuel cell 30.

The CO₂ adsorbing member 20 ₂ adsorbs CO₂ molecules in the fed exhaustgas, and decomposes them into carbon monoxide molecules and oxygenatoms. That is, the decomposition and reduction reaction expressed byEq. (3) described above (CO₂→CO+1/20₂) occurs in the CO₂ adsorbingmember 20 ₁ as well.

In the configuration illustrated in FIG. 3, CO₂ adsorbing member 20 ₁may be formed of multiple flat plate-shaped members stacked in layers atpredetermined intervals or be a cylindrical member as described below ora prism-shaped member. The same applies to the CO₂ adsorbing member 20₂.

FIG. 4 illustrates a detailed configuration of the CO₂ adsorbing member20 ₁. A description of the CO₂ adsorbing member 20 ₂, which has the sameconfiguration of the CO₂ adsorbing member 20 ₁, is omitted.

Referring to FIG. 4, the CO₂ adsorbing member 20 ₁ includes a base 21and a CO₂ adsorbent 22 of, for example, 1 nm to 1 μm in film thicknessformed on the base 21. The base 21 is preferably a porous body and has alarge surface area. The CO₂ adsorbent 22 is a metal oxide film that hasoxygen deficiencies 23 and preferably has a perovskite structure such asSrTiO₃ or PZT. For example, in the case where the CO₂ adsorbent 22 isformed of SrTiO₃, the CO₂ adsorbent 22 generally has a nonstoichiometriccomposition expressed by a chemical formula of Sr_(1-x)Ti_(1-y)O_(3-z),while in the application to the fuel cell system 100 of FIG. 1, it ispreferable that the composition parameter z be more than or equal to 0.1to include a large number of oxygen deficiencies in particular. Further,in addition to SrTiO₃ and PZT, examples of the CO₂ adsorbent 22 mayinclude other metal oxides having the perovskite structure, such asBaTiO₃, BaSrTiO₃, CaTiO₃, and CaMnO₃. Further, fired porous silica ordiatomaceous earth may be used as the base 21.

Here, referring to the flowchart of FIG. 2, in a process of (a), theexhaust gas containing a CO₂ molecule 24 is fed into the containers 10 ₁and 10 ₂ forming the CO₂ adsorption and decomposition tank 40 ₁ and theCO₂ adsorption and decomposition tank 40 ₂, respectively. In thefollowing, a description is given of the CO₂ adsorption anddecomposition tank 40 ₁, and a description of the CO₂ adsorption anddecomposition tank 40 ₂, which is the same as the description of the CO₂adsorption and decomposition tank 40 ₁, is omitted.

Then, as illustrated in a process of (b) of FIG. 2, one of the oxygenatoms of the CO₂ molecule 24 is captured with (due to) the oxygendeficiency 23. That is, in the process of (b), the CO₂ molecule 24 isadsorbed by the CO₂ adsorbent 22.

In the processes of (a) and (b) of FIG. 2, the CO₂ adsorbent 22 is setat room temperature by feeding air to the heat exchanger 12 ₁. Theprocess of (b), however, may be executed at any temperature between roomtemperature and 100° C. Hereinafter, the process of (a) may be referredto as an “introduction process,” and the process of (b) may be referredto as an “adsorption process.”

The pumps 36 ₁ and 36 ₂ (FIG. 1) are provided for the CO₂ adsorption anddecomposition tanks 40 ₁ and 40 ₂, respectively, for introduction of theexhaust gas and for other processes described below. For example, in theintroduction process of FIG. 2, the pump 36 ₁ is driven to evacuate thecontainer 10 ₁ of the CO₂ adsorption and decomposition tank 40 ₁, sothat the exhaust gas that has passed through the H₂O separator 32 isintroduced into the CO₂ adsorption and decomposition tank 40 ₁.

Next, in a process of (c) of FIG. 2, the temperature of the CO₂adsorbent 22 is increased to 200° C. to 300° C., and the pressure of theprocessing space in which the CO₂ adsorbent 22 is provided is reduced toapproximately 0.01 atm. to approximately 0.1 atm. using the pump 36 ₁.As a result, the CO₂ molecule 24 is decomposed into the oxygen atomcaptured due to the oxygen deficiency 23 and a CO molecule 25, so thatCO is liberated from CO₂. That is, CO₂ is reduced to CO, so that CO isextracted from CO₂. The CO gas thus extracted is collected into the COstorage tank 33 as described above. For example, the pump 36 ₁ is drivento send the extracted CO gas from the container 10 ₁ of the CO₂adsorption and decomposition tank 40 ₁ to the CO storage tank 33, sothat the extracted CO gas is collected into the CO storage tank 33.Hereinafter, the process of (c) may be referred to as a “decompositionprocess.”

In the decomposition process, the CO₂ adsorbent 22 of the CO₂ adsorptionand decomposition tank 40 ₁ is adjusted to the above-describedtemperature of 200° C. to 300° C. by feeding the high-temperatureexhaust gas from the fuel cell 30 to the heat exchanger 12 ₁ whilecontrolling the flow rate of the high-temperature exhaust gas. If thefuel cell 30 is an SOFC, the high-temperature exhaust gas has atemperature of approximately 800° C. If the fuel cell 30 is an MCFC, thehigh-temperature exhaust gas has a temperature of approximately 700° C.

Further, when substantially all of the CO₂ molecules 24 adsorbed by theCO₂ adsorbent 22 are decomposed, in a process of (d) of FIG. 2, thecontainer 10 ₁ is evacuated by the pump 36 ₁, and the CO₂ adsorbent 22is heated to preferably the temperature of the high-temperature exhaustgas or a slightly lower temperature of 600° C. to 700° C. As a result,the oxygen atom captured due to the oxygen deficiency 23 (as well asother oxygen atoms captured with the oxygen deficiencies 23) is releasedto be removed from the CO₂ adsorption and decomposition tank 40 ₁ in theform of an oxygen molecule (O₂) 26. Hereinafter, the process of (d) maybe referred to as a “regeneration process.”

In the regeneration process, the CO₂ adsorbent 22 of the CO₂ adsorptionand decomposition tank 40 ₁ may be heated to the above-describetemperature of 600° C. to 700° C. by feeding the high-temperatureexhaust gas from the fuel cell 30 to the heat exchanger 12 ₁ at amaximum flow rate.

Then, the processing returns to the process of (a) and theabove-described processes (a) through (d) are repeated. Thereby, CO isrepeatedly extracted from CO₂ in the exhaust gas of the fuel cell 30 tobe reused as fuel. As a result, it is possible to significantly improvethe energy balance of the fuel cell 30.

Likewise, the same “introduction process,” “adsorption process,”“decomposition process,” and “regeneration process” are also executed inthe CO₂ adsorption and decomposition tank 40 ₂, although theirdescription is omitted. It is preferable that the CO₂ adsorption anddecomposition tanks 40 ₁ and 40 ₂ be caused to operate alternately inthe fuel cell system 100 of FIG. 1.

That is, as illustrated in the flowchart of FIG. 5, while the CO₂adsorption and decomposition tank 40 ₁ is caused to operate at roomtemperature (RT) so as to execute the introduction process and theadsorption process that do not require feeding of the high-temperatureexhaust gas of the fuel cell 30 to the heat exchanger 12 ₁, thedecomposition process and the regeneration process are executed in theCO₂ adsorption and decomposition tank 40 ₂ with feeding of thehigh-temperature exhaust gas of the fuel cell 30 to the heat exchanger12 ₂. On the other hand, while the CO₂ adsorption and decomposition tank40 ₂ is caused to operate at room temperature (RT) so as to execute theintroduction process and the adsorption process that do not requirefeeding of the high-temperature exhaust gas of the fuel cell 30 to theheat exchanger 12 ₂, the decomposition process and the regenerationprocess are executed in the CO₂ adsorption and decomposition tank 40 ₁with feeding of the high-temperature exhaust gas of the fuel cell 30 tothe heat exchanger 12 ₁.

By thus causing the CO₂ adsorption and decomposition tanks 40 ₁ and 40 ₂to operate alternately, it is possible to make efficient use of the heatof the high-temperature exhaust gas discharged from the fuel cell 30, sothat it is possible to execute the decomposition process and theregeneration process with high energy efficiency without using externalenergy or generated electric power.

Therefore, the fuel cell system 100 of FIG. 1 has valves Va₁ through Vd₁and Vg₁ and the valves Ve₁ and Vf₁ provided in a first exhaust gasprocessing system including the CO₂ adsorption and decomposition tank 40₁, and has valves Va₂ through Vd₂ and Vg₂ and the valves Ve₂ and Vf₂provided in a second exhaust gas processing system including the CO₂adsorption and decomposition tank 40 ₂.

A description is given below, with reference to FIG. 1, of the valvesVa₁ through Vg₁.

The valve Va₁ is provided between an exhaust opening 30 e of the fuelcell 30 and an inlet/outlet port 12 a ₁ of the heat exchanger 12 ₁ ofthe CO₂ adsorption and decomposition tank 40 ₁. Correspondingly, on theopposite side of the heat exchanger 12 ₁, the valve Vc₁ for feeding theexhaust gas that has passed through the heat exchanger 12 ₁ to anexhaust gas inlet port 31 a ₁ of the heat exchanger 31 ₁ is provided foran outlet/inlet port 12 b ₁ of the heat exchanger 12 ₁. The exhaust gasof the fuel cell 30 is also fed directly from the exhaust port 30 e tothe exhaust gas inlet port 31 a ₁ of the heat exchanger 31 ₁.

Further, in the CO₂ adsorption and decomposition tank 40 ₁, the valveVg₁ for feeding the air in the air line 35 a to the heat exchanger 12 ₁is provided between the air line 35 a and the outlet/inlet port 12 b ₁of the heat exchanger 12 ₁. Further, the valve Vb₁ for feeding the airthat has passed through the heat exchanger 12 ₁ to an air inlet port 31c ₁ of the heat exchanger 31 ₁ is provided for the inlet/outlet port 12a ₁ of the heat exchanger 12 ₁. The air inlet port 31 c ₁ is connectedto the air line 35 a so that air is fed to the air inlet port 31 c ₁.

When the valves Va₁ and Vc₁ are opened, part of the exhaust gasdischarged from the exhaust port 30 e of the fuel cell 30 passes throughthe heat exchanger 12 ₁ of the CO₂ adsorption and decomposition tank 40₁ to be fed to the exhaust gas inlet port 31 a ₁ of the heat exchanger31 ₁ and added to the exhaust gas fed directly from the exhaust port 30e to the exhaust gas inlet port 31 a ₁.

Further, the valves Vg₁ and Vb₁ are opened to cause the air in the airline 35 a to be fed to air inlet port 31 c ₁ through the heat exchanger12 ₁. Then, after being heated, the air is fed from an air outlet port31 d ₁ on the opposite side of the heat exchanger 31 ₁ to the airelectrode 30 ₁ of the fuel cell 30. At this point, in the CO₂ adsorptionand decomposition tank 40 ₁, the temperature of the CO₂ adsorbing member20 ₁ is set at room temperature.

Further, according to the fuel cell system 100 of FIG. 1, the exhaustgas is fed from an exhaust gas outlet port 31 b ₁ of the heat exchanger31 ₁ to the H₂O separator 32 formed of a cold trap, where water isseparated. Thereafter, as described above, the exhaust gas is fed to theCO₂ adsorption and decomposition tank 40 ₁ through the valve Vf₁. In theCO₂ adsorption and decomposition tank 40 ₁, the exhaust gas is fed tothe exhaust gas feed part 11 ₁ provided to the CO₂ adsorbing member 20₁.

Further, the CO gas extracted in the CO₂ adsorption and decompositiontank 40 ₁ in the above-described decomposition process ((c) of FIG. 2)is fed to the CO storage tank 33 through the pump 36 ₁ and the valveVe₁, and is collected and stored in the CO storage tank 33.

Further, the O₂ gas released in the CO₂ adsorption and decompositiontank 40 ₁ in the above-described regeneration process ((d) of FIG. 2) isdischarged outside the fuel cell system 100 through the pump 36 ₁ andthe valve Vd₁.

Likewise, the valves Va₂ through Vg₂ are provided in the second exhaustgas processing system including the CO₂ adsorption and decompositiontank 40 ₂ in the fuel cell system 100.

The valves Va₂ through Vg₂ correspond to the valves Va₁ through Vg₁,respectively, and accordingly, a description thereof is omitted.

Tables 1 through 4 below illustrate an opening and closing sequence ofthe valves Va₁ through Vg₁ and the valves Va₂ through Vg₂ correspondingto the flowchart of FIG. 5. The following opening and closing sequenceof the valves Va₁ through Vg₁ and the valves Va₂ through Vg₂ iscontrolled by a controller 101 schematically illustrated in FIG. 1. Itis preferable that the controller 101 be implemented by a computerloaded with a control program.

TABLE 1 [CO₂ Adsorption and Decomposition Tank 40₁: IntroductionProcess, CO₂ Adsorption and Decomposition Tank 40₂: DecompositionProcess] Va₁ Vb₁ Vc₁ Vd₁ Ve₁ Vf₁ Vg₁ CLOSED OPEN CLOSED OPEN CLOSED OPENOPEN Va₂ Vb₂ Vc₂ Vd₂ Ve₂ Vf₂ Vg₂ OPEN CLOSED OPEN CLOSED OPEN CLOSEDCLOSED

TABLE 2 [CO₂ Adsorption and Decomposition Tank 40₁: Adsorption Process,CO₂ Adsorption and Decomposition Tank 40₂: Regeneration Process] Va₁ Vb₁Vc₁ Vd₁ Ve₁ Vf₁ Vg₁ CLOSED OPEN CLOSED CLOSED CLOSED CLOSED OPEN Va₂ Vb₂Vc₂ Vd₂ Ve₂ Vf₂ Vg₂ OPEN CLOSED OPEN OPEN CLOSED CLOSED CLOSED

TABLE 3 [CO₂ Adsorption and Decomposition Tank 40₁: DecompositionProcess, CO₂ Adsorption and Decomposition Tank 40₂: IntroductionProcess] Va₁ Vb₁ Vc₁ Vd₁ Ve₁ Vf₁ Vg₁ OPEN CLOSED OPEN CLOSED OPEN CLOSEDCLOSED Va₂ Vb₂ Vc₂ Vd₂ Ve₂ Vf₂ Vg₂ CLOSED OPEN CLOSED OPEN CLOSED OPENOPEN

TABLE 4 [CO₂ Adsorption and Decomposition Tank 40₁: RegenerationProcess, CO₂ Adsorption and Decomposition Tank 40₂: Adsorption Process]Va₁ Vb₁ Vc₁ Vd₁ Ve₁ Vf₁ Vg₁ OPEN CLOSED OPEN OPEN CLOSED CLOSED CLOSEDVa₂ Vb₂ Vc₂ Vd₂ Ve₂ Vf₂ Vg₂ CLOSED OPEN CLOSED CLOSED CLOSED CLOSED OPEN

Referring to Table 1, in the introduction process of the CO₂ adsorptionand decomposition tank 40 ₁, the valves Vb₁ and Vg₁ are open with thevalves Va₁, Vc₁, and Ve₁ being closed. As a result, the air in the airline 35 a is fed to the heat exchanger 12 ₁ of the CO₂ adsorption anddecomposition tank 40 ₁, so that the temperature of the CO₂ adsorbingmember 20 ₁ is set at room temperature. In this state, the valve Vd₁ isopened and the pump 36 ₁ is driven so as to evacuate the container 10 ₁in the CO₂ adsorption and decomposition tank 40 ₁ so that the pressureinside the container 10 ₁ (the processing space) is reduced toapproximately 0.01 atm. to approximately 0.1 atm. In this state, thevalve Vf₁ is opened to allow the exhaust gas that has passed through theH₂O separator 32 to flow into the container 10 ₁ and fill in the CO₂adsorption and decomposition tank 40 ₁.

Next, in the adsorption process of the CO₂ adsorption and decompositiontank 40 ₁, the valves Va₁, Vc₁, and Ve₁ are kept closed while the valvesVb₁ and Vg₁ remain open, so that the air in the air line 35 a continuesto be fed to the heat exchanger 12 ₁ of the CO₂ adsorption anddecomposition tank 40 ₁ and the temperature of the CO₂ adsorbing member20 ₁ is kept at room temperature. Further, in this state, the valves Vd₁and Vf₁ are closed, and the CO₂ gas introduced into the container 10 ₁is adsorbed by the CO₂ adsorbent 22 of the CO₂ adsorbing member 20 ₁.

Next, in the decomposition process of the CO₂ adsorption anddecomposition tank 40 ₁, the valves Va₁ and Vc₁ are open while thevalves Vb₁ and Vg₁ are closed. As a result, the exhaust gas of the fuelcell 30 is introduced into the heat exchanger 12 ₁, so that thetemperature of the CO₂ adsorbing member 20 ₁ is set at 200° C. to 300°C. In this state, while keeping the valves Vd₁ and Vf₁ closed, the valveVe₁ is opened and the pump 36 ₁ is driven, so that the CO gas extractedin the CO₂ adsorption and decomposition tank 40 ₁ is fed to the COstorage tank 33. In the decomposition process, the temperature of theCO₂ adsorbing member 20 ₁ may be set at 200° C. to 300° C. bycontrolling the degree of opening of the valves Va₁ and Vc₁.

Next, in the regeneration process of the CO₂ adsorption anddecomposition tank 40 ₁, while keeping the valves Va₁ and Vc₁ open andkeeping the valves Vb₁, Vf₁, and Vg₁ closed, the valve Ve₁ is closed andthe valve Vd₁ is opened. Further, the pump 36 ₁ is driven to reduce thepressure of the processing space in the container 10 ₁ to approximately0.01 atm. to approximately 0.1 atm. As a result, the oxygen atomsadsorbed to the CO₂ adsorbing member 20 ₁ are released into theprocessing space to be released outside the fuel cell system 100 asoxygen gas. As a result, the CO₂ adsorbing member 20 ₁ is regenerated inthe CO₂ adsorption and decomposition tank 40 ₁. The oxygen gas thusproduced may be returned to the air line 35 a to be re-supplied to theair electrode 30 ₁ of the fuel cell 30 instead of being dischargedoutside the fuel cell system 100.

Referring to Table 1, during the introduction process in the CO₂adsorption and decomposition tank 40 ₁, the decomposition process isexecuted to decompose CO₂ in the CO₂ adsorption and decomposition tank40 ₂ with the valves Vb₂, Vd₂, Vf₂, and Vg₂ being closed and the valvesVa₂, Vc₂, and Ve₂ being open.

Referring to Table 2, during the adsorption process in the CO₂adsorption and decomposition tank 40 ₁, the regeneration process isexecuted to regenerate the CO₂ adsorbing member 20 ₂ in the CO₂adsorption and decomposition tank 40 ₂ with the valves Vb₂, Ve₂, Vf₂,and Vg₂ being closed and the valves Va₂, Vc₂, and Vd₂ being open.

Referring to Table 3, during the decomposition process in the CO₂adsorption and decomposition tank 40 ₁, the introduction process isexecuted to introduce the exhaust gas that has passed the H₂O separator32 into the container 10 ₂ in the CO₂ adsorption and decomposition tank40 ₂ with the valves Va₂, Vc₂, and Ve₂ being closed and the valves Vb₂,Vd₂, Vf₂, and Vg₂ being open.

Referring to Table 4, during the regeneration process in the CO₂adsorption and decomposition tank 40 ₁, the adsorption process isexecuted for the CO₂ adsorbing member 20 ₂ to adsorb CO₂ in the CO₂adsorption and decomposition tank 40 ₂ with the valves Va₂, Vc₂, Vd₂,Ve₂, and Vf₂ being closed and the valves Vb₂ and Vg₂ being open.

Thus, according to this embodiment, a CO₂ adsorption and decompositiontank having a simple structure is employed together with a simple heatfeeding cycle in order to decompose CO₂. As a result, it is possible toconstruct a fuel cell system having a simple structure and to facilitatethe maintenance of the CO₂ adsorption and decomposition tanks 40 ₁ and40 ₂.

Further, according to this embodiment, high-temperature exhaust gas froman SOFC or an MCFC is used as thermal energy for decomposing CO₂ andregenerating oxygen holes. Therefore, no extra heating part is necessaryor only a small amount heat may be applied for such purposes.Accordingly, it is possible to achieve high energy balance.

For example, decomposing 1 g of CO₂ into CO needs approximately 500 J ofenergy. According to the fuel cell system 100 of FIG. 1, although theefficiency is reduced because of various losses caused by systemintegration, it is possible to decompose 1 g of CO₂ with approximately1000 J of energy. Further, according to this embodiment, since thedecomposition of CO₂ and the regeneration of the CO₂ adsorbing members20 ₁ and 20 ₂ are thermal reactions, there is no particular necessityfor external energy, and waste heat from the fuel cell 30 may be usedfor the reactions. Accordingly, the fuel cell system 100 of FIG. 1 issignificantly higher in efficiency than the conventional method usingelectrolysis.

Further, according to the fuel cell system 100 of FIG. 1, CO₂ in theexhaust gas is reduced not as far as to hydrogen or a hydrocarbon but toCO, which is producible with less conversion energy. In this respect aswell, it is possible to improve energy balance. Further, according tothe fuel cell system 100 of FIG. 1 of this embodiment, CO₂-containingexhaust gas from an SOFC or an MCFC is recycled as part of rawmaterials. Accordingly, it is possible to construct an SOFC fuel cellsystem or an MCFC fuel cell system reduced in CO₂ emission.

FIG. 6 is a flowchart illustrating an operation of the fuel cell system100 of FIG. 1 according to this embodiment.

If the cycle illustrated in FIG. 2 is repeated in the fuel cell system100 of FIG. 1, practically, the amount of carbon in the fuel cell system100, and accordingly, the amount of CO₂ discharged from the fuel cell 30gradually increase with a new input of fuel gas. Therefore, theadsorption process of (b) of FIG. 2 does not decompose all CO₂ in theexhaust gas with the CO₂ adsorbing members 20 ₁ and 20 ₂. Therefore, inpractice, after the introduction process (step S1) and the subsequentadsorption process (step S2), a discharging process (step S21) ispreferably performed before proceeding to the decomposition process(step S3) as illustrated in FIG. 6. In the discharging process, excessCO₂ is discharged outside the fuel cell system 100 by opening the valvesVd₁ and Vd₂ and driving the pumps 36 ₁ and 36 ₂ while keeping thetemperature of the CO₂ adsorbing members 20 ₁ and 20 ₂ at roomtemperature.

When the valve Vd₁ is open in the discharge process of step S21, theother valves Va₁, Vb₁, Vc₁, Ve₁, Vf₁, and Vg₁ that cooperate with theCO₂ adsorption and decomposition tank 40 ₁ are kept closed. Likewise,when the valve Vd₂ is open in the discharge process of step S21, theother valves Va₂, Vb₂, Vc₂, Ve₂, Vf₂, and Vg₂ that cooperate with theCO₂ adsorption and decomposition tank 40 ₂ are kept closed.

The discharge process of step S21 of FIG. 6 may be incorporated into theadsorption process in the timing chart of FIG. 5.

A description is given below in another embodiment of processing theexcess CO₂ thus discharged.

When the introduction process is entered after the regeneration process,the subsequent adsorption process does not progress normally unless theCO₂ adsorbing members 20 ₁ and 20 ₂ are sufficiently low in temperature,for example, in the range of room temperature to 100° C. It may bepossible to wait for the CO₂ adsorbing members 20 ₁ and 20 ₂ to becooled by natural air cooling. For better operational efficiency of thefuel cell system 100, however, it is preferable to provide a coolingprocess (step S41) after the regeneration process (step S4) asillustrated in FIG. 6. In the cooling process of step S41, thetemperature of the CO₂ adsorbing member 20 ₁ or 20 ₂ may be reduced to avalue in the above-described predetermined range by feeding air to theheat exchanger 12 ₁ or 12 ₂ of the CO₂ adsorption and decomposition tank40 ₁ or 40 ₂ by closing the valves Va₁, Vc₁, Ve₁, and Vf₁ and openingthe valves Vb₁, Vd₁, and Vg₁ or closing the valves Va₂, Vc₂, Ve₂, andVf₂ and opening the valves Vb₂, Vd₂, and Vg₂.

The cooling process of step S41 of FIG. 6 may be incorporated into theregeneration process in the timing chart of FIG. 5.

In this embodiment, a description is given of the case of providing thetwo CO₂ adsorption and decomposition tanks 40 ₁ and 40 ₂. However, thenumber of CO₂ adsorption and decomposition tanks provided is not limitedto two, and three or more CO₂ adsorption and decomposition tanks may beprovided.

[b] Second Embodiment

FIG. 7 is a diagram illustrating a configuration of the CO₂ adsorbingmember 20 ₁ used in the CO₂ adsorption and decomposition tank 40 ₁ inthe fuel cell system 100 of FIG. 1 according to a second embodiment. Theconfiguration of the CO₂ adsorbing member 20 ₂ used in the CO₂adsorption and decomposition tank 40 ₂ in this embodiment may be thesame as the configuration of the CO₂ adsorbing member 20 ₁ illustratedin FIG. 7, and accordingly, a description thereof is omitted.

Referring to (a) of FIG. 7, a CO₂ adsorbing member 40 includes acylindrical porous base body 41 formed by molding and firing poroussilica powder and a container 45 of stainless steel or the like thathouses the porous base body 41. Multiple exhaust gas passages 42 areformed in the porous base body 41. The exhaust gas passages 42 may beapproximately 2 mm in diameter, for example.

As illustrated in an enlarged view of (b) of FIG. 7, multiple recesses46 such as minute air gaps of approximately several nm to approximatelyseveral tens of μm in diameter are formed in the base body 41, and aSrTiO₃ film of approximately 1 nm in thickness is formed on the surfacesof the recesses 46 as a CO₂ adsorbent layer 43, for example. The CO₂adsorbent layer 43 is preferably a ceramic film formed by a film formingmethod that provides good step coverage, such as CVD or a sol-gelprocess, and has a film thickness of approximately 1 nm to approximately1 μm. If the CO₂ adsorbent layer 43 is too thick, the recesses 46 areclosed. Accordingly, it is preferable that the CO₂ adsorbent layer 43have a thickness within the above-described range. It is preferable thatthe recesses 46 illustrated in the enlarged view of (b) of FIG. 7 formsuccessive open pores on the entire surface of the porous base body 41.

Further, multiple gas pipes 44 that operate as the heat exchanger 12 ₁(FIG. 3) are inserted through corresponding openings 44A in the porousbase body 41. By causing air from the air line 35 a or exhaust gas fromthe fuel cell 30 (FIG. 1) to flow through the gas pipes 44, thetemperature of the CO₂ adsorbing member 40 may be controlled to be apredetermined value as described above.

In the CO₂ adsorbing member 40 of FIG. 7, the (cylindrical) side surfaceand the end faces of the porous base body 41 are covered with thecontainer 45.

In addition to the SrTiO₃ film mentioned above, examples of the CO₂adsorbent layer 43 may include films of other oxides having theperovskite structure, such as BaTiO₃, PZT, and PLZT. These perovskitefilms are generally polycrystal ceramic films in the case of formingthem on the porous base body 41 formed by molding and firing poroussilica powder. On the other hand, these perovskite films are monocrystalfilms in the case of using, for example, a thin monocrystal substrate ofSrTiO₃ as the porous base body 41.

FIG. 8 is a diagram illustrating a CO₂ adsorbing member 40A according toa variation of the second embodiment.

The CO₂ adsorbing member 40A may be formed by providing multiple thincircular monocrystal substrates 41S at intervals in the container 45 ofFIG. 7 as illustrated in FIG. 8.

Referring to FIG. 8, the monocrystal substrates 41S having the exhaustgas passages 42 (FIG. 7) formed therein are spaced apart from oneanother in the container 45. Exhaust gas is fed to an inlet 45 aprovided in a cap 45A at one end of the container 45, and is dischargedfrom an outlet 45 b provided in a cap 45B at the other end of thecontainer 45. Further, the gas pipes 44 (FIG. 7) are provided in thecontainer 45 to serve as a heat exchanger.

Further, in the configuration of FIG. 8, the porous base body 41 formedof a fired body illustrated in FIG. 7 may be used for the substrates41S. According to this configuration, it is possible to form the CO₂adsorbent layer 43 on a thin porous base body, thus allowing the CO₂adsorbent layer 43 to be formed easily by CVD or ALD.

[c] Third Embodiment

FIG. 9, FIG. 10, FIG. 11, and FIG. 12 are diagrams illustrating anoperation sequence of the fuel cell system 100 according to a thirdembodiment. In FIG. 9 through FIG. 12, the elements corresponding tothose described above are referred to by the same reference numerals,and a description thereof is omitted. Further, for simplification, adescription is given of the first exhaust gas processing systemincluding the valves Va₁ through Vg₁.

Referring to (a) and (b) of FIG. 9 as well as FIG. 1, at this stage inthis embodiment, the valves Va₁ through Vg₁ are set the same as in theintroduction process in the CO₂ adsorption and decomposition tank 40 ₁of Table 1, and the pump 36 ₁ is driven to introduce the exhaust gasfrom the H₂O separator 32 into the CO₂ adsorption and decomposition tank40 ₁.

At this point in this embodiment, not only the introduction process ofstep S1 illustrated in FIG. 6 but also the adsorption process of step S2and the excess CO₂ discharging process of step S21 are performed in thestate of FIG. 9. During this period, the valve Vd_(i) is open and thepump 36 ₁ is driven in this embodiment. Therefore, excess CO₂ that hasnot been adsorbed by the CO₂ adsorbing member 20 ₁ in the CO₂ adsorptionand decomposition tank 40 ₁ is discharged outside the fuel cell system100.

Next, referring to (a) and (b) of FIG. 10 as well as FIG. 1, the valvesVa₁ through Vg₁ are set the same as in the decomposition process in theCO₂ adsorption and decomposition tank 40 ₁ of Table 3, and the CO₂adsorbing member 20 ₁ is heated to a temperature of 200° C. to 300° C.in the CO₂ adsorption and decomposition tank 40 ₁ and extracted CO gasis collected into the CO storage tank 33. In the state of FIG. 10, theexhaust gas that has passed through the heat exchangers 31 ₁ and 31 ₂ isfed to the CO₂ adsorption and decomposition tank 40 ₂.

Then, referring to (a) and (b) of FIG. 11 as well as FIG. 1, the valvesVa₁ through Vg₁ are set the same as in the regeneration process in theCO₂ adsorption and decomposition tank 40 ₁ of Table 4, and the CO₂adsorbing member 20 ₁ is heated to a temperature of 600° C. to 700° C.in the CO₂ adsorption and decomposition tank 40 ₁ and released O₂ gas isdischarged outside the fuel cell system 100 through the valve Vd₁. Inthe state of FIG. 11 as well, the exhaust gas that has passed throughthe heat exchangers 31 ₁ and 31 ₂ is fed to the CO₂ adsorption anddecomposition tank 40 ₂.

Then, referring to (a) and (b) of FIG. 12 as well as FIG. 1, the valvesVa₁, Vc₁, Ve₁, and Vf₁ are closed and the valves Vb₁, Vd₁, and Vg₁ areopened, and air is fed to the heat exchanger 12 ₁, so that the CO₂adsorbing member 20 ₁ is cooled to room temperature to 100° C. in theCO₂ adsorption and decomposition tank 40 ₁.

According to this embodiment, the introduction process, the adsorptionprocess, and the excess CO₂ discharging process may be performed whilekeeping the valves Va₁ through Vg₁ in the same state as illustrated inFIG. 9. This facilitates operating the fuel cell system 100.

[d] Fourth Embodiment

FIG. 13 is a diagram illustrating a fuel cell system 100A according to afourth embodiment. FIG. 14 is a diagram illustrating one of multiplealgae culturing tanks 63 provided in the fuel cell system 100A of FIG.13 to culture algae to serve as a raw material of bioethanol. In FIG.13, the same elements as those described above are referred to by thesame reference numerals.

Referring to FIG. 13, according to this embodiment, a cooling jacket 61through which a coolant such as cooling water flows is attached to thefuel cell 30, and the multiple algae culturing tanks 63 are provided incorrespondence to the cooling jacket 61. Further, according to thisembodiment, multiple thermoelectric conversion elements 62 are providedinside the cooling jacket 61 to be in contact with the body of the fuelcell 30 and the interior surface of the cooling jacket 61, so as togenerate electric power using the waste heat of the fuel cell 30.

Thermoelectric modules using, for example, bismuth-telluride (Bi—Te)system materials or lead-telluride (Pb—Te) system materials may beemployed as the thermoelectric conversion elements 62.

Use of these thermoelectric conversion elements 62 makes it possible tofurther improve the energy efficiency of the fuel cell system 100A.

FIG. 14 illustrates one of the algae culturing tanks 63.

Referring to FIG. 14, the algae culturing tank 63 includes a tank body63A, an annular nozzle member 63 b including multiple nozzles, andagitating blades 65. The tank body 63A stores water to culture algae.The nozzle member 63 b is submerged in the water stored in the tank body63A, and is supplied, through an exhaust gas line 63 a, with exhaust gascomposed principally of CO₂ discharged from the valve Vd₁ or Vd₂ anddischarges the exhaust gas into the tank body 63A through the nozzles.The agitating blades 65 are driven by a motor 65A to agitate the waterin the tank body 63A. For example, the motor 65A is driven with part ofthe electric power generated in the thermoelectric conversion elements62.

Further, the tank body 63A is provided with a feed water line 66A forfeeding water to the tank body 63A from outside and a drainage line 66Bfor discharging the water in the tank body 63A.

Further, the warm water discharged from the fuel cell 30 flows through adischarge water pipe 64 in the tank body 63A to be used for adjusting(controlling) the temperature of the water in the tank body 63A.

Thus, according to this embodiment, CO₂ discharged from the fuel cell 30is used to culture algae, and the cultured algae are used as a rawmaterial of bioethanol. As a result, it is possible to reduce CO₂released to the environment and to contribute to prevention of globalwarming.

In the embodiment illustrated in FIG. 13, the three algae culturingtanks 63 are provided for the single fuel cell 30. However, the numberof algae culturing tanks 63 provided is not limited to three, and may beone, two, or more than three.

Thus, according to an aspect of the invention, CO₂ discharged from afuel cell may be reduced to CO with a simple configuration withoutinputting external energy as in the conventional electrolysis process,and high energy balance may be achieved in a fuel cell system by reusingthe CO as part of fuel. Further, the amount of CO₂ emitted as a resultof electric power generation may be reduced by generating electric powerusing the waste heat of the fuel cell and/or by culturing algae to serveas a raw material of biofuel such as bioethanol using CO₂ that has notbeen reused, so that it is possible to contribute to prevention ofglobal warming.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventors to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority orinferiority of the invention. Although the embodiments of the presentinventions have been described in detail, it should be understood thatvarious changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

1. A fuel cell system, comprising: a fuel cell configured to generate electric power with a fuel gas and an oxygen gas fed thereto and to discharge an exhaust gas including CO₂ as a result of generating the electric power; a CO extraction part configured to reduce the CO₂ in the exhaust gas fed thereto to CO, the CO extraction part including a processing container fed with the exhaust gas and a CO₂ adsorbing member provided in the processing container and formed of an oxide having an oxygen deficiency; and a CO recycling part configured to feed the extracted CO to the fuel cell as a part of the fuel gas.
 2. The fuel cell system as claimed in claim 1, wherein the oxide has a perovskite structure.
 3. The fuel cell system as claimed in claim 1, wherein the CO extraction part further includes: a heating part configured to heat the CO₂ adsorbing member.
 4. The fuel cell system as claimed in claim 3, wherein the heating part is configured to heat the CO₂ adsorbing member with the exhaust gas.
 5. The fuel cell system as claimed in claim 3, wherein the heating part is configured to heat the CO₂ adsorbing member to one of a first temperature of 200° C. to 300° C. and a second temperature of 600° C. to 700° C.
 6. The fuel cell system as claimed in claim 1, wherein the CO extraction part further includes: a cooling part configured to cool the CO₂ adsorbing member to a room temperature.
 7. The fuel cell system as claimed in claim 1, wherein the CO extraction part further includes: a pump configured to evacuate the processing container.
 8. The fuel cell system as claimed in claim 1, wherein the CO recycling part includes: a tank configured to retain the CO generated by the reduction in the CO extraction part; and a CO gas adding part configured to add the CO in the tank to the fuel gas.
 9. The fuel cell system as claimed in claim 1, further comprising: a culturing tank configured to be fed with the exhaust gas from the CO extraction part, the exhaust gas including a remaining portion of the CO₂ unreduced in the CO extraction part, and to culture algae using the remaining portion of the CO₂ as a nutrient.
 10. A method of controlling a fuel cell system, comprising: adsorbing a CO₂ molecule in an exhaust gas by a CO₂ adsorbing member at a first temperature, the exhaust gas being discharged from a fuel cell as a result of generating electric power in the fuel cell; desorbing a CO molecule including a first oxygen atom from the CO₂ adsorbing member by increasing a temperature of the CO₂ adsorbing member from the first temperature to a second temperature higher than the first temperature; desorbing a second oxygen atom remaining in the CO₂ adsorbing member from the CO₂ adsorbing member by increasing the temperature of the CO₂ adsorbing member from the second temperature to a third temperature higher than the second temperature; and cooling the CO₂ adsorbing member to the first temperature, wherein said adsorbing the CO₂ molecule, said desorbing the CO molecule, said desorbing the oxygen atom, and said cooling the CO₂ adsorbing member are sequentially performed during reduction of CO₂ in the exhaust gas to CO.
 11. The method as claimed in claim 10, further comprising: feeding the CO extracted by the reduction to the fuel cell as a part of a fuel gas fed to the fuel cell.
 12. The method as claimed in claim 10, wherein the CO₂ adsorbing member adsorbs the CO₂ molecule by capturing the second oxygen atom of the CO₂ molecule with an oxygen deficiency of the CO₂ adsorbing member in said adsorbing the CO₂ molecule.
 13. The method as claimed in claim 12, wherein the CO₂ adsorbing member decomposes the CO₂ molecule into the second oxygen atom and the CO molecule while retaining the second oxygen atom captured with the oxygen deficiency by being heated to the second temperature in said desorbing the CO molecule.
 14. The method as claimed in claim 13, wherein the CO₂ adsorbing member releases the second oxygen atom captured with the oxygen deficiency by being heated to the third temperature in said desorbing the second oxygen atom.
 15. The method as claimed in claim 10, wherein: the reduction of the CO₂ in the exhaust gas to the CO is performed in at least a first system and a second system of the fuel cell system, the first system and the second system each including the CO₂ adsorbing member, and when said adsorbing the CO₂ molecule is performed in the first system, an operation of the second system is changed sequentially from said desorbing the CO molecule to said desorbing the oxygen atom to said cooling the CO₂ adsorbing member, and when said adsorbing the CO₂ molecule is performed in the second system, an operation of the first system is changed sequentially from said desorbing the CO molecule to said desorbing the oxygen atom to said cooling the CO₂ adsorbing member. 